Pass mime type values as file dialog accept filters (see #3314)

File dialogs that specify mime type (e.g. "image/*") accept filters will pass
those values unchanged to the OnFileDialog |accept_filters| parameter. The
default dialog implementation will show those filters in addition to a combined
"Custom Files" filter. This is a change from preexisting Google Chrome
behavior where only the combined "Custom Files" filter is displayed, and
restores CEF behavior that existed prior to 2ea7459a89.

Document the fact that OnFileDialog may be called twice, once before MIME type
expansion and once afterwards.

Add new OnFileDialog |accept_extensions| and |accept_descriptions| parameters
for MIME type extensions and descriptions.

Details: This change adds a SelectFileDialog::FileTypeInfo::extension_mimetypes
member and improves the logic in FileSelectHelper::GetFileTypesFromAcceptType
and file_dialog_manager.cc SelectFileToFileChooserParams to support recall of
the source mime type when populating the FileChooserParams structure.

To test:
- Run `ceftests --gtest_filter=DialogTest.*`
- Run `cefclient --url=https://tests/dialogs`

tests/cefclient/browser/dialog_handler_gtk.h

@@ -25,6 +25,8 @@ class ClientDialogHandlerGtk : public CefDialogHandler,
                     const CefString& title,
                     const CefString& default_file_path,
                     const std::vector<CefString>& accept_filters,
+                    const std::vector<CefString>& accept_extensions,
+                    const std::vector<CefString>& accept_descriptions,
                     CefRefPtr<CefFileDialogCallback> callback) override;
 
   // CefJSDialogHandler methods.
@@ -48,6 +50,8 @@ class ClientDialogHandlerGtk : public CefDialogHandler,
     CefString title;
     CefString default_file_path;
     std::vector<CefString> accept_filters;
+    std::vector<CefString> accept_extensions;
+    std::vector<CefString> accept_descriptions;
     CefRefPtr<CefFileDialogCallback> callback;
   };
   void OnFileDialogContinue(const OnFileDialogParams& params,